Failure investigation of fatigue cracks initiated at business jet vertical tail angles

At one of the scheduled inspections conducted during a business jet full scale fatigue testing, three surface cracks were detected at the radius of drag angles that attach the vertical tail and the Aft fuselage. The main role of these angles is to support the structural response to the vertical tail yaw rotation during flight, and it is not considered as a principal structural element. However, it appears that the angles were also contributed to carrying the vertical tail bending loads during the simulated flight. This additional loading, in which the drag angles were not designed to, was the main trigger for crack nucleation at the drag angle radius. As part of a root cause analysis, both drag angles were instrumented by means of strain gauges to validate the finite element predictions. These measurements were further verified by means of photoelastic stress measurements that also account for strain and stress distributions rather than the localized strain gauge measurements. This manuscript presents the root cause investigation that was carried out as well as the suggested repair.